1. Field of the Invention
The present invention relates to the treatment of injection and producing oil and gas wells and, more particularly, to the removal of residual polymeric material from such wells.
2. Background of the Description
In the recovery of oil from underground formations, it is often necessary to resort to enhanced recovery techniques since the formation pressure rarely remains sufficient enough to force the oil to flow freely up the primary well bore. One technique commonly employed in enhanced recovery involves polymer flooding. In this technique, other wells, commonly referred to as injection wells, are drilled into the oil bearing formation in generally surrounding relationship to the primary well bore. An aqueous flooding medium containing a polymeric material is then pumped under pressure into the injection wells, the aqueous polymeric medium serving to dislodge the oil in the formation solids and force it towards the primary well bore for recovery.
While polymer flooding is a widely used and successful method of enhanced oil recovery, it is not without its difficulties. As is well known, polymer flooding is accomplished by injecting the aqueous flooding medium containing the dissolved or dispersed polymer into the formation in perforations which extend laterally out from the injection wells. Typically, a desired longitudinal segment of the injection well is chosen as the perforated zone, this zone being effectively packed off from the remainder of the well bore such that the flooding operation can take place without the loss of excessive flooding medium in the injection well.
As the polymer flooding progresses, it is common for solid polymeric material to deposit on the walls of the well bore in the perforated zone and in the interstices of the formation adjacent the perforated zone. Indeed, this polymer deposition will eventually form a plug which markedly interferes with the injection of the polymer flooding medium through the perforations. This necessitates the use of higher and higher pressures in order to inject the aqueous flooding medium and eventually becomes so severe as to prevent further flooding operations.
Polymeric build-up plugging can also occur in the primary well bore i.e. the producing well, in as much as the aqueous medium containing the polymeric material once it passes through the formation, is forced into the producing well. As in the case of the injection wells, the polymeric buildup occurs primarily in the perforated zone of the producing well and in the interstices of the formation immediately surrounding the perforated zone.
In an attempt to remove or at least reduce these polymeric build-ups or plugs, it is known to use oxidants such as hydrogen peroxide, sodium hypochlorite and sodium perborate. The use of hydrogen peroxide or sodium perborate is undesirable because it is relatively expensive. On the other hand, sodium hypochlorite while it is relatively inexpensive, is an inefficient oxidant in degrading the polymeric material.
Canadian Pat. No. 1,207,269 discloses a method of treating oil field produced fluids with chlorine dioxide which acts as a biocide, an emulsion breaker and a hydrogen sulfide scavenger. The Canadian patent teaches that the chlorine dioxide can be injected into the water or other oil field produced brine before or after a first stage separation of the oil from the other fluids and also that in tertiary recovery operations it can be injected before the first stage separation to break emulsions which form in polymer flooding. Chlorine dioxide has also been used in small quantities for wells as a biocide and hydrogen sulfide scavenger.